Gateway support "raw" serial device as sensor network

[ghost]

Add support in Gateway for communicating MQTT-SN over a standard serial device, without any additional protocol between.
E.g. XBee uses a raw serial device beneath but adds the XBee protocol before using MQTT-SN on top.

This issue aims for connecting a sensor device via USB (or other serial link) directly to the computer running the MQTTSNGateway.
E.g. an Arduino that connects to a RaspberryPI which runs the Gateway, and uses MQTT-SN as protocol between them.